To properly secure an input power supply to an inverter device when an AC power supply is lost and then recovered, by increasing the secondary side voltage of a transformer with an output voltage variable function by a specific value as compared with a rated voltage when the AC power supply is lost and then recovered, and returning it to a rated voltage after a specific amount of time passes.

An AC power supply 1 is connected to a transformer 5 with an output voltage variable function and is transformed to a specific voltage. On the other hand, a plurality of inverter devices 3a-3n are connected to a load system and motors 4a-4n are connected to each of them. In this case, when the power of the AC power supply 1 fails temporarily and then recovers, a control device 6 switches the tap of the transformer 5 with an output voltage variable function to increase the output voltage by a specific value as compared with a rated voltage, and switches a tape again after a specific amount of time passes and returns to the rated voltage, thus enabling the inverters 3a-3n to properly drive motors 4a-4n when the AC power supply 1 is lost and then recovered.